udhcpc: account for script run time

udhcpc: account for script run time

Based on the following user report:

I ran into an issue where I was seeing a long delay in the scripts called
in udhcp_run_script. I was using an old version of OpenWrt (kamikaze)
and a satellite modem. An NTP script was being called and the modem
would sometimes take a long time to respond to the DNS lookup when
it was offline.

This delay started affecting my lease time. The lease that I would
get from my satellite modem before it was online would be short:
only 60 seconds. The delay with NTP and the modem would typically
be about 18 seconds. This would cause the first DHCP renew request
from dhcpc to be a little late. Under certain circumstances,
I could even see the first DHCP renew to occur after the lease
had expired!

function                                             old     new   delta
udhcpc_main                                         2816    2837     +21

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>